1. Field of the Invention
The present invention relates to piezoelectric filter to be suitably used for a filter circuit of mobile radio telecommunication equipment such as a portable mobile radio transmitter/receiver unit or an automobile telephone set.
2. Related Background Art
A piezoelectric filter of the type under consideration typically comprises a multiple of unit filter circuits, each having one or more than one series resonators and one or more than one parallel resonators connected to form an L-connection and arranged between an input terminal and an output terminal or a grounding terminal. The resonance frequency of the series resonators and the antiresonance frequency of the parallel resonators are made to correspond with each other and the difference between the antiresonance frequency of any of the series resonators and the resonance frequency of any of the parallel resonators is held to a constant value. These and other ideas have been proposed for piezoelectric filters comprising a plurality of resonators in order to improve the performance and reduce the required number of piezoelectric filters. An example of such conventional piezoelectric filters is disclosed in Examined Japanese UM Publication No. 57-12579.
However, in most conventional piezoelectric filters having a configuration as described above, resonators and terminal plates are typically assembled to a multilayer structure within a casing and hence they inevitably comprise a large number of components to entail a cumbersome assembling process. Meanwhile, piezoelectric filters are often required to be as thin as several millimeters to meet the trend of downsizing. Additionally, they are required to be configured in such a way that each of the piezoelectric filters can be electrically connected with relative ease to a predetermined electric path arranged on a plane that carries resonators thereon and the resonators are electrically and mechanically protected.
Still additionally, piezoelectric filters should be easy to manufacture and assemble.